Occupancy of the iron binding sites of human transferrin.

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Abstract
The in vivo distribution of Fe between the binding sites of transferrin was examined. Plasma was obtained from normal subjects under basal conditions and after in vitro and in vivo Fe loading. Independent methods, including measurement of the transferrin profile after isoelectric focusing and cross immunoelectrophoresis, and determination of the Fe content in the separated fractions were in agreement that there was a random distribution of Fe on binding sites. This held true with in vitro loading, when Fe was increased by intestinal absorption and with loading from the RES. The distribution of apo-, monoferric and diferric transferrins is predictable on the basis of the plasma transferrin saturation and negate the concept that Fe loading of transferrin in vitro is a selective process with possible functional consequences in tissue Fe delivery.